1. Field of Invention
The present invention relates to a multi-lamp driving system for driving fluorescent lamps, such as, for example, cold cathode fluorescent lamps (CCFLs), and more particularly to the drive topology of such circuits.
2. Description of Related Arts
Fluorescent lamps are used to provide illumination for general lighting purposes. The critical factors in the design of a cold cathode fluorescent lamp (CCFL) include efficiency, cost, and size. CCFLs (cold cathode fluorescent lamps) are wildly employed in display panels as a backlight. Generally speaking, CCFLs require approximately 1500 Volts (RMS) to strike, and require approximately 800 Volts (RMS) for steady state operation. In displays where two CCFLs are required, a conventional technique is couple the lamps in parallel with the secondary side of a step-up transformer. In multi-lamp system, the conventional technique for driving the lamps is to couple the lamps together in parallel with one another to the transformer. While this ensures, voltage control during striking, this topology also requires impedance matching circuitry for the lamps. Also, current control in this topology is difficult since the current conditions of each lamp must be monitored.
FIG. 1 shows a conventional dual lamp circuit having two CCFLs coupled in parallel, which includes a DC power supply 101, a DC/AC converter 102, a step-up transformer 103, an impedance matching network 104, a first CCFLs 105, and a second CCFL 106. The DC/AC converter 102 could be any one type of a full-bridge circuit, a half-bridge circuit, a push-pull circuit or a Royer circuit. The power supply 101 is coupled to the input of the DC/AC converter 102. In other words, the power supply 101 provides DC power to the DC/AC converter 102. The DC/AC converter 102 converts the DC power to AC power. The AC power is used to supply to a primary side of the step-up transformer 103. A secondary side of the step-up transformer 103 is coupled to the impedance matching network 104. The impedance matching network 104 comprises a high voltage inductor and two high voltage capacitors to deliver distributed current to each CCFL. However, it is hard to design appropriate an inductance of the inductor and capacitances of two high voltage capacitors in order to deliver evenly distributed current to each CCFL. Furthermore, in a multi-lamp system, the system needs more inductors to achieve the current s to deliver distributed current to each CCFL and consequently cause the complexity in design.
FIG. 2 shows another conventional driving circuit for driving a plurality of cold cathode fluorescent lamps. DC power 201 is supplied to a DC/AC converter 202. The DC/AC converter 202 converts the DC power to AC power, wherein the DC/AC converter 202 could be any one type of a full-bridge circuit a half-bridge circuit, a push-pull circuit or a Royer circuit. The AC power is used to supply to a primary side of the step-up transformer 203. The secondary winding of the transformer 203 is coupled to three cold cathode fluorescent lamps 207, 209, and 209 through three high voltage capacitors 204, 205, and 206, respectively. Any adjacent two cold cathode fluorescent lamps are connected to a corresponding common-mode choke CC.sub. 1 to CC.sub.2. In other words, when the DC/AC converter circuit structure of the present invention is used to drive a plurality of cold cathode fluorescent lamps, the number of used common-mode chokes used is less than the number of the driven cold cathode fluorescent lamps by one. The common-mode choke balances the current flowing through each lamp so that each lamp provides same amount of luminance, as is described in U.S. Pat. No. 6,781,325. However, there are many drawbacks in the conventional driving circuit. For example, in order to implement open circuit protection of a CCFL, the prior art generally utilizes a series resistor connected to the lower voltage side of the lamp to detect a lamp current, however, it is difficult to achieve an open circuit protection of a cold cathode fluorescent lamp because there exists an induced voltage across the series resistor whether the lamp current passes through the CCFL or not. In other words, there exists an induced voltage across the series resistor even though the CCFL is open. Therefore, it is difficult to detect whether the CCFL is under normal operation or not. Furthermore, when the CCFL is open, there exists an induced high voltage across one of the windings of the common-mode choke because the common-mode choke will try to balance the current flowing through each lamp. Therefore, the induced high voltage probably harms the common-mode choke or the peripheral circuits.